Potentiometer or the like



April 28, 1970 w. l. ELLIOTT laiTAl.A 3,508,329

' POTENTIOMETEROR THE LIKE original Filed Jan. 24, 196e INVENTORS MLA/,4M 2L/orf 32 BY c7464( L. PAA/04u United States Patent O Int. Cl. H01c 1/14 U-S. Cl. 29-619 5 Claims ABSTRACT F THE DISCLOSURE A method of making a potentiometer, including the steps of:

(1) Winding an insulation-coated resistance wire onto a cylindrical support with adjacent turns in abutment;

(2) securing the confronting portions of the insulation and the support together;

(3) removing the insulation'along the outer portion of each turn between the portions of insulation separating adjacent turns;

(4) forming a split plastic ring having a smooth inner surface of an axial length suicient to span a plu,- rality of turns of the wire;

(5 placing the plastic ring around the wire;

(6) deforming the inner surface of thel plastic ring to form alternate grooves and ribs, wherein the grooves conform to the contour of the exposed wire of the turns and the ribs nest between adjacent turns, the turns and ribs being in mesh so as to coact, upon rotating the support while holding the plastic ring against rotation, to effect longitudinal movement of the plastic element;

(7) and attaching an electrical contact element to the plastic ring, with a portion of such element in wiping engagement with the exposed wire on at least one turn.

This application is a division of application Ser. No. 522,736, tiled Jan. 24, 1966, now Patent No. 3,399,368.

This invention relates to apparatus using variable resistance devices, and more particularly to a unique construction for potentiometers and the like with which to obtain infinite resolution with a wire-wound resistor.

It has long been known toapply a voltage across the terminals of a resistor element formed of many turns of resistance wire, and to move an electrical wiper contact, or pick-off, from turn to turn to change the voltage between either terminal and the wiper. The greater the number of turns, the smaller is the increment of change in voltage from turn to turn. Thus, a resistor may be formed of 2,500 turns per inch of wire wound on a mandrel, in which case movement of the wiper along the mandrel between adjacent turns etfects a change of 1/5500, or 0.0004 of the total voltage across either terminal of the resistor and the wiper. But this is the smallest voltage change that can be effected, ordetected, with this arrangement.

However small the wire, and however large the number of turns, it has not heretofore been possible with such wire-wound resistors to obtain infinite resolution, i.e., to effect as small a voltage change as desired. In many complex electronic systems, it is often desirable to change a voltage applied to a circuit by as little as a millionth of the applied voltage.

Variable resistors with which infinite resolution can be obtained have been made with powdered resistive and ceramic materials mixed together and sintered to form a monolithic element. In some, the proportions of ingredients in the mixture are the same throughout the length of the element, but the thickness of the element varies lCe from end to end. In others, the element is of uniform thickness along its length, but the quantity of resistive material varies `gradually from end to end. In either case, the wiper can be moved to any desired position on the element to effect any desired voltage change.

However, there are certain disadvantages in the use of such resistor elements. Due to their particulate structure, they manifest relatively high electrical noise levels and temperature coefficients of resistance, which are greatly improved by resistors wound with wire of solid metal.

It is an object of our invention to provide wire-wound resistor apparatus with which to achieve infinite resolution.

It is another object of our invention to provide a unique wire-wound resistor and wiper therefor which function both as electrical elements and as cooperative threaded means to make the wiper follow the turns of the resistor.

A further object of our invention is to provide a potentiometer or like structure having a minimum number of component parts of simple design and dugged construction.

The above and other objects and advantages of our invention will become apparent from the following description taken in conjunction with the accompanying drawing of an illustrative embodiment thereof, in which:

FIGURE 1 is a longitudinal sectional View of a potentiometer of our invention, wherein the windings of the resistor element constitute threads for mating engagement with threads of an element which functions both as the wiper and a traveling nut;

FIGURE 2 is a sectional view taken along the lines 2 2 of FIGURE 1, showing more clearly the arrangement of the wiper-nut element;

FIGURE 3 is a fragmentary sectional view taken along the lines 3 3 of FIGURE 1, to aid in explaining the movement of the wiper-nut along the resistor element;

FIGURE 4 is an enlarged fragmentary sectional view of a portion of the resistor element encircled by the wiper-nut, to aid in explaining the method of our invention in forming the cooperative threads; and

FIGURE 5 is an enlarged fragmentary sectional view of the wiper-nut at one end of the resistor element at which further linear movement thereof is prevented, to aid in explaining the operations of the mating parts to prevent stripping of the threads thereof.

Referring to FIGURES 1 and 2, a potentiometer 10 in v accordance with our invention employs a housing 11 having a central opening 12 therein, in which a mandrel 13 is adapted to rotate. The mandrel 13 carries a resistance winding 14 on its outer surface, the turns of which form mating threads with a plastic nut 15, which is formed as a broken ring, and which is supported in a spring clip 16 which at one end forms a conductive wiper contact 17 engaging the winding 14. The spring clip 16 is provided with a projection 18 that rides in the groove or keyway 19 formed in an elongated channel-shaped member 20. The channel 20 is of conductive material, and is electrically connected at one end to an external pin 21, and the ends of the winding 14 are connected to conductive slip rings 22, 23 which rotate with the mandrel 13, and which, through respective wiper contacts 24, 25, are electrically connected to external pins 26, 27.

In this manner, rotation of the mandrel 13, and hence the resistor winding 14, effects longitudinal movement of the nut 15, the direction of travel of the nut depending upon the direction of rotation of the mandrel 13. In this manner, the wiper 17 is caused to follow the turns of the vwinding 14 as it moves from one to the other thereof. Accordingly, any desired adjustment in the position of the nut 15 can be made to effect any desired change in voltage at the pin 21.

For example, assume the winding 24 to be one inch Patented Apr. 28, 1970 f long, and to comprise 2500 turns of wire, the ends of which are connected, via terminal pins 26, 27, to a source of voltage (not shown). One revolution of the mandrel 13 causes the wiper 17 to traverse the length of wire which accounts for 1/2500 or 0.0004, of the total applied voltage. However, inasmuch as the wiper engages such turn throughout such a revolution, a fraction of the revolution effects a corresponding fractional change in the portion of the voltage represented by a complete turn. Thus, moving the mandrel 13 through an angle of one degree effects a change of 1/360 of the 0.0004 fraction of the applied voltage, or approximately one-millionth of such applied voltage. Movement of the mandrel through a fraction of a degree effects a correspondingly smaller fractional change of the applied voltage.

It will be appreciated that it is impossible by conventional methods to make a threaded element with threads of as slight a pitch as above indicated. Even by engraving methods, it would be next to impossible to form threads of the order of a 0.0004 inch pitch with accuracy. Referring to FIGURE 4, in our invention we utilize the resistor winding 14 for forming the internal threads of the nut 15. To this end, we rst wind the resistance wire on the mandrel 13, it being understood that such wire, however small it is in diameter, is coated with insulation 30. The turns of the winding 14 are attached to the mandrel 13, as by a coating 31 of plastic material, such as an epoxy resin, which forms a bond between the mandrel 13 and the confronting portions of the insulation 30 around the wire.

In this latter connection, the winding may be attached to the mandrel in any desired manner. For example, the thin coating of the plastic material may be applied to the mandrel before winding the wire thereon. The wire is wound on the mandrel so that the insulation coatings on adjacent turns are in abutment. If the plastic coating 31 is hardened at the time the winding is wound thereon, the assembly is placed in an oven and heated to cause the plastic material to flow and fill the helical groove that is defined by the diverging portions of the insulation 30 and the outer surface of the mandrel. 1f the coating 31 is Wet when the wire is wound on the mandrel, the plastic material fills such groove, whereupon subsequent curing of the plastic material causes the winding to be rmly bonded to the mandrel.

Further, the winding may be wound on a dry, i.e., uncoated, mandrel. The ends of the mandrel are plugged, and it is immersed in a container of liquid material to be used for bonding. The contaner is then placed in a vacuum chamber, and a vacuum is drawn to cause the liquid to be drawn between the mandrel and the winding, and to fll all interstices therebetween. The assembly is then removed from the vacuum and the bonding material is allowed to harden.

After the winding 14 is bonded to the mandrel 13, the outer portion of the insulation 30 around each turn of wire is removed. Removal of the insulation from the wire can be effected by any known technique, as by soda bufiing, wherein an abrasive of finely divided particles of soda are applied to the external portions of the insulation by a sand blasting technique. Alternatively, the insulation may be removed by simply rubbing it off with a soft eraser.

For purposes of a precision potentiometer, the mandrel 13 is of sufficiently large diameter as to avoid undue stress in the Wire by virtue of wrapping it around a cylinder. As will be appreciated, winding any Wire on a mandrel that is too small in diameter requires that the Wire follow such a short radius as to set up stresses therein, which would be greatest where the wire engages the mandrel. The effects of such stresses become pronounced over wide temperature variations, and result in marked changes in the temperature coefficient and the resistance of the wire in different temperature ranges. Where such changes cannot be tolerated, we employ a mandrel that is of suliicient outside diameter as to substantially eliminate this possibility. For example, in one application using Wire of 0.0004 inch diameter, we employ a mandrel having an outside diameter of approximately 0.25 inch.

Also, where the apparatus is subjected to a wide range of temperatures, the material used for the wire preferably has a temperature coefficient which is as nearly like that of the mandrel as possible. The mandrel may be made of brass, which has an extremely low temperature coeiiicient, in which case the material used for the wire of the winding 14 should be made of alloys of metal having a comparable coefficient of expansion and contraction.

As previously mentioned, the nut 15 is formed of plastic material. If the material of the nut 15 is one that is capable of being deformed by the application of pressure alone, e.g., Teflon, the spring clip 16 surrounding the material is utilized for this purpose. /Since the elements forming the wiper and the nut are split rings, the constant inward pressure of the spring clip 16 results in grooves 32 (FIG. 4) which conform to the exposed portions of the wire, and ribs 33 formed of the portion of the material that is forced inwardly toward the portions of insulation 30 between adjacent turns of the wire.

As will be seen, this arrangement results in the formation of internal threads for the nut 15 which matingly engage the turns of the winding, so that rotation of the mandrel 13 effects linear travel of the nut 15 in a manner conventional for linear actuators.

Where the nut 15 is to be formed of a plastic material which is extremely hard and has long-wearing qualities, e.g., a nylon molding compound, the force exerted by the clip 16 is insufficient to deform the internal surface of the nut to provide the desired internal threads therefor. In such case, the split ring for the nut is molded, and then inserted in the clip 16 and bonded thereto with a suitable plastic material. Such bonding may be facilitated by roughening the outer surface of the molded ring, as by etching, before bonding it to the spring clip 16. The clip 16 and the plastic ring therein are snapped onto the exterior of the winding 14. We then heat the internal surface of the plastic ring to a suliicient temperature to cause the material to flow, whereupon the pressure exerted by the spring clip 16 forces the material into the grooves between adjacent turns of the wire, thereby to form the internal threads for the nut as previously described in connection with FIGURE 4.

The application of heat for molding the threads in the inner surface of such a hard plastic element may be effected in a variety of ways. One technique is to apply a heating current through the wires of the winding 14 of suiiicient magnitude to heat the internal surface of the element. Due to the continuing inward pressure exerted by the clip element 16, the softening of the inner surface of the element forces it inwardly to form the grooves and ribs 32, 33 as shown in FIG- URE 4.

Another technique for carrying out this heating step is to place the entire assembly in an oven and heat to a sufficient temperature to effect-the desired molding of the threads.

Molding of the threads on the nut 15 may be accomplished on a volume basis. To this end, we employl a master mandrel wound with a resistance winding as described herein, and position a plurality of assembled plastic rings in spring clips around the Winding. The heating step ypreviously described thus forms internal threads on all of the plastic nut elements simultaneously. After su`ch molding, each of the wiper-nutv assemblies is removed from the master unit and placed on a respective Wire-wound mandrel, on which wire of the same diameter has been wound.

For a fuller understanding of the operation of the potentiometer of our invention, reference is made again to FIGURES 1-3. As shown, the housing 11 is formed as an elongated cylinder having an end cap 40 bonded in place as by a suitable plastic adhesive coated on the confronting surfaces of the cylinder and the end cap. The housing parts may be formed of any desired material, e.g., metal or plastic.

As shown at the right-hand end of the housing in FIGURE 1, the end cap 40 has an inner cylindrical projection 41 which is surrounded by a metallic sleeve 42 forming a sleeve bearing. Surrounding the bearing 42 is a metallic sleeve 43, which is press-fit into a cylinder 44 which extends into the end of the mandrel 13. The confronting surfaces of the mandrel 13 and the cylinder 44 are bonded together, as with a suitable epoxy cement. The flange 45 of the cylinder 44 abuts the ends of the mandrel 13, and extends radially a substantial distance beyond the winding 14. As shown, the slip ring 23 is mounted on the periphery of the ange 45, to which it is bonded. The adjacent end of the winding 14 is suitably connected to the slip ring 23, as by extending it through a groove in the radial face of the flange 45, and soldering or welding it at 46 to the inner surface of the slip ring 23.

At the opposite end of the mandrel 13, the slip ring 22 is mounted on the periphery of the flange 48` of a cylinder 49 that extends into and is bonded to the interior surface of the mandrel 13, in the manner of the cylinder 44. The outer diameter of the flange 48 is t-he same as that of the flange 45, and the slip ring 22 is the same size as the slip ring 23. As in the case of the terminal of the winding connected to the slip ring 23, the end of the winding adjacent the ange 48 is similarly connected at 50 to the slip ring 22.

The cylinder 49 is press-fit on the inner end of a stub shaft 51 that extends to the exterior of the housing 11. As shown, the stub shaft 51 is rotatably mounted in a bushing 52 which is secured in the left end of the housing. The outer end of the stub shaft 51 may be provided with a screwdriver slot 53 through which to rotate the mandrel manually, or the stub shaft may be adapted as at 54 for operation by a motor 55. Thus, it will be seen that the shaft 51, the mandrel 13 'with the winding 14 thereon, and the slip rings 22, 23 are arranged for unitary rotation.

As seen in FIGURES 1 and 2, the Wiper contact 25 extends into a groove S6, where it is soldered or welded to one end of a wire 57 extending the length of the groove from the inner end of the terminal pin 27. As shown, the wire 57 is laid in the bottom of the groove 56, and is potted in place, by lling the groove with a suitable plastic material, indicated at 58. At the right end of the housing, the wiper contact 26 extends into a similar groove S9, where it is soldered or welded to a wire 60 extending from the inner end of the terminal pin 26, as in the case of the wire 57 and terminal pin 27. Also, as in the case of the potting material 58 in the groove 56, the groove 59 is filled with potting material 61.

To aid in maintaining the desired electrical contact between the Winding 14 and the terminal pin 21, a short metallic leaf spring 62 is soldered or welded to the projection 18. As shown, the leaf spring 62 is soldered to the radially extending portion of the projection 18, and is bent over the upper end of the projection. The spring 62 is self-biased outwardly from the end of the projection 18, so as to be maintained in continual engagement with the inner surface of the web portion of the channel The Wiper-nut construction in our invention offers the unique advantage that when the nut engages either of the flanges 45, 48 at the ends of the mandrel, continued rotation of the mandrel in the same direction will not result in damage to the cooperating threads of the winding 14 and the nut 15. In other words, our invention dispenses with the requirement in prior art linear actuators for mechanical slip clutches and/or electromagnetlc clutches operated by switches disposed in the paths of the traveling elements.

Referring to FIGURE 5 along with FIGURE 1, 'when the nut 15 is brought to bear against the flange 45, the split ring conguration of the clip 16 and the nut 15 permit the threads constituted of the winding 14 to cam the nut 15 and clip 16 outwardly during those portions of the revolution in which the threads would, with a nut formed of a solid ring, result in stripping of the threads or alternately, causing the interfering threads to bind and thereby place an overload on the motor v55. By this means, if the mandrel 13 continues to turn in the same direction, the nut 15 simply opens and closes as long as the mandrel is turned, but without damage to any of the parts of the potentiometer or the actuating means therefor. As soon as the direction of rotation of the mandrel is reversed, the threads of the nut immediately resume mating engagement with the turns of the winding, to cause travel of the nut in the opposite direction.

To facilitate the above-described operation of the wipernut arrangement, and to avoid any short-circuiting problerns, we make the axial length of the nut and clip 16 greater than the axial dimensions of the projection 18 and the portion of the spring wiper 62 that is welded or soldered thereto. Additionally, the outer diameter of the metal clip 16 is smaller than the inner diameter of the slip rings 22, 23. Thus, when the nut reaches the end of its travel at either end of the mandrel, none of the metal parts associated with the clip 16 can engage a slip ring.

It will be noted that we have illustrated the nut as spanning several turns of the winding 14. For a nut of given axial dimension, the number of turns of the winding spanned thereby, i.e., the number of threads formed in the nut, Will depend upon the size of the wire. For wire of the order of 0.0004-in. diameter, a nut of 0.080-in. axial length spans of the order of 200 turns of the winding 14. For a winding 14 wound with wire of 0.0l6-in. diameter, a nut of such axial dimension spans 'five (5) turns. Correspondingly, of course, the wiper portion I17 of the clip 16 spans a number of turns of the winding, the number of which also depends upon the size of the wire used. We -nd that this arrangement is benecial, in that it insures mechanical stability of the nut, i.e., by preventing tilting thereof as it is moved along the mandrel.

From the foregoing, it will be apparent that various modications can be made in the method and apparatus as illustrated and described herein, without departing from the spirit and scope of our invention. Accordingly, we do not intend that our invention be limited, except as by the appended claims.

We claim:

1. The method of making variable resistance apparatus comprising the steps of:

winding an insulation-coated resistance wire onto a cylindrical support with adjacent turns in abutment; securing the confronting portions of the insulation and the support together;

removing the insulation along the outer portion of each turn between the portions of insulation separating adjacent turns;

forming a split plastic ring having a smooth inner surface of an axial length sucient to span a plurality of turns of the Wire;

placing the plastic ring around the wire;

deforming the inner surface of the plastic ring to form alternate grooves and ribs, wherein the grooves conform to the contour of the exposed wire of the turns and the ribs nest between adjacent turns, said turns and ribs being in mesh so as to coact, upon rotating the support while holding the plastic ring against rotation, to effect longitudinal movement of the plastic element;

and attaching an electrical contact element to the plastic ring, with a portion of such element in wiping engagement with the exposed wire on at least one turn.

2. The method of claim 1, wherein the deforming step includes applying pressure radially inwardly on the plastic ring, the material of the ring being deformable by the application of pressure alone.

3. The method of claim 1, wherein the material of the plastic ring is deformable upon application of heat and pressure, and wherein the deforming step includes heating the inner surface of the plastic ring, and applying pressure radially inwardly on the ring while the inner surface thereof is heated.

4. The method of claim 3, including passing an electric current through the Wire to heat the inner surface of the plastic ring.

5. The method of claim 4, including forming a split ring clip of conductive spring metal:

inserting the plastic ring within and securing it to the clip prior to placing it around the wire, the clip eX- 15 erting inward radial pressure on the plastic ring, the

ends of the clip and the plastic ring being Hush and spaced apart; and securing a spring wiper contact to the clip, with one end of such contact extending between the ends of the slip and slidably contacting the wire on the support.

' References Cited UNITED STATES PATENTS 0 2,873,509 2/1959 sorber 29-610 1 3,069,646 12/1962 Hardison et al. 338-143 3,402,464 9/1968 Hatch 29-610 JOHN P. CAMPBELL, Primary Examiner U.S. Cl. X.R. 338-143 

